Primary lithium electrochemical cell

ABSTRACT

A primary electrochemical cell includes a cathode including lambda-manganese dioxide (γ-MnO 2 ), an anode including lithium or a lithium alloy, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte contacting the anode and the cathode.

TECHNICAL FIELD

[0001] This invention relates to a primary lithium electrochemical celland a method of manufacturing a primary lithium electrochemical cell.

BACKGROUND

[0002] A battery includes one or more galvanic cells (i.e., cells thatproduce a direct current of electricity) in a finished package. Cells ofthis type generally contain two electrodes separated by a medium capableof transporting ions, called an electrolyte. Typical electrolytesinclude liquid organic electrolytes or a polymeric electrolytes. Thecell produces electricity from chemical reactions through oxidation atone electrode, commonly referred to as the negative electrode or anode,and reduction at the other electrode, commonly referred to as thepositive electrode or cathode. Completion of an electrically conductingcircuit including the negative and positive electrodes allows iontransport across the cell and discharges the battery. A primary batteryis intended to be discharged to exhaustion once, and then discarded. Arechargable battery can be discharged and recharged multiple times.

[0003] An example of a primary battery is a primary lithium cell. Alithium electrochemical cell is a galvanic cell using lithium, a lithiumalloy or other lithium containing materials as one electrode in thecell. The other electrode of the cell can include, for example, atransition metal oxide, such as gamma-manganese dioxide (γ-manganesedioxide or γ-MnO₂) or transition metal sulfide such as iron disulfide.The metal oxide or sulfide used in the electrode can be processed priorto use in a lithium battery. Generally, γ-manganese dioxide can beprepared by chemical methods or electrochemical methods. The resultingmaterials are known as chemically produced γ-manganese dioxide (CMD) andelectrochemically produced (e.g., electrolytic or electrodeposited)γ-manganese dioxide (EMD), respectively.

SUMMARY

[0004] A primary electrochemical cell includes a cathode includinglambda-manganese dioxide (γ-MnO₂) having a spinel-related crystalstructure. The anode can include lithium metal or a lithium alloy, suchas lithium aluminum alloy.

[0005] In one aspect, a primary lithium electrochemical cell includes acathode including lambda-manganese dioxide, an anode including lithium,a separator between the anode and the cathode, and an electrolytecontacting the cathode, the anode and the separator. The cell has anaverage closed circuit voltage (CCV) between 3.8 and 4.1V and a specificdischarge capacity to a 3V cutoff of greater than about 130 mAh/g at anominal discharge rate of 1 mA/cm². The cell can have a 3V cutoff ofgreater than 135 mAh/g or 140 mAh/g or greater at a nominal dischargerate of 0.4 mA/cm².

[0006] The lambda-manganese dioxide can be heated to a temperature ofless than 150° C., or 120° C. or less, during processing and cathodefabrication.

[0007] In another aspect, a method of preparing lambda-manganese dioxideincludes contacting water with a compound having the general formulaLi_(1+x)Mn_(2−x)O₄, wherein x is from −0.02 to +0.02, or −0.005 to+0.005, adding an acid to the mixture of water and the compound untilthe water has a pH of 1 or less, preferably between 0.5 and 1,separating a solid product from the water and acid, and drying the solidat a temperature of 150° C. or less to obtain the lambda-manganesedioxide. The method can include washing the solid separated from thewater and acid with water until the washings have a pH of between 6 and7.

[0008] The compound can have a BET surface area of between 1 and 10m²/g, or greater than 4 m²/g or greater than 8 m²/g, a total pore volumeof between 0.02 and 0.2, or 0.05 and 0.15, cubic centimeters per gram,or an average pore size of between 100 and 300 Å.

[0009] The solid can be dried at a temperature between 20° C. and 120°C., 30° C. and 90° C., or between 50° C. and 70° C. A vacuum optionallycan be applied during drying.

[0010] Contacting water and the compound includes forming a slurry. Theslurry can be maintained at a temperature below 50° C. or between about10° C. and 50° C., or about 15° C. to 30° C. The acid can be sulfuricacid, nitric acid, perchloric acid, hydrochloric acid, toluenesulfonicacid or trifluoromethylsulfonic acid. The acid solution can have aconcentration between 1 and 8 molar. The temperature of the slurry canbe held substantially constant during the addition of the acid.

[0011] In another aspect, a method of manufacturing a primaryelectrochemical cell includes providing a positive electrode includinglambda-manganese oxide and forming a cell including the positiveelectrode and a negative electrode including lithium. The cell has aclosed circuit voltage between 3.8V and 4.1V and a specific dischargecapacity to a 3V cutoff of greater than about 130 mAh/g at a nominaldischarge rate of 1 mA/cm². Providing the electrode can includepreparing lambda-manganese dioxide by a method including contactingwater with a compound of the formula Li_(1+x)Mn_(2−x)O₄, wherein x isfrom −0.02 to +0.02, adding an acid to the water and compound until thewater has a pH of 1 or less, separating a solid from the water and acid,and drying the solid at a temperature of 150° C. or below to obtain thelambda-manganese dioxide. The electrode can be fabricated by mixing thelambda-manganese dioxide with a conductive additive and an optionalbinder.

[0012] A primary lithium electrochemical cell including a cathodecontaining γ-MnO₂ can have an average closed circuit voltage of between3.8V and 4.1V, a specific discharge capacity to a 3V cutoff of greaterthan 135 mAh/g at a discharge rate of 1 mA/cm², good high-rateperformance, and adequate capacity retention when stored. A closedcircuit voltage of about 4V can provide desirable voltage compatibilitywith lithium-ion secondary cells having cathodes containing LiCoO₂,LiNiO₂ or solid solutions thereof (i.e. LiCo_(x)Ni_(1−x)O₂, wherein0<x<1). A specific single cycle capacity of greater than 135 mAh/g canprovide greater capacity than the average single cycle capacity for atypical lithium-ion secondary cell having a cathode containing LiCoO₂,LiNiO₂ or solid solutions thereof. Adequate capacity retention whenstored can be especially important because in a primary electrochemicalcell any loss of capacity cannot be recovered through recharging. Aprimary lithium electrochemical cell having a cathode including γ-MnO₂can have a higher total energy density than a primary lithiumelectrochemical cell having a cathode including heat-treated γ/β-MnO₂and having an average closed circuit voltage of about 2.8V.

[0013] The physical and chemical properties of a lithium manganese oxidespinel powder (LiMn₂O₄) used as a precursor for the γ-MnO₂, especiallythe chemical stoichiometry and the particle microstructure, candramatically influence the 4V discharge capacity and the thermalstability of the resulting γ-MnO₂ product. A high-capacity γ-MnO₂ can beproduced by substantially completely removing of lithium from the spinellattice of a nominally stoichiometric precursor spinel, for example, bytreating the spinel with acid to a pH value of less than 2. By avoidingheat treatment of the γ-MnO₂ powder above 150° C., for example in therange of 80° C. to 120° C., decomposition of the γ-MnO₂ can be reducedor avoided, leading to a specific capacity for a low-rate discharge ofgreater than 130 mAh/g to a 3V cutoff, but less than the theoretical 4Vcapacity of about 154 mAh/g.

[0014] Other features and advantages of the invention will be apparentfrom the description of the preferred embodiments and from the claims.

DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a cross-section view of a wound lithium primaryelectrochemical cell.

[0016]FIG. 2 is a graph depicting a comparison of x-ray powderdiffraction patterns for γ-MnO₂ powders and the nominally stoichiometricspinel precursor powder.

[0017]FIG. 3 is a graph depicting the discharge performance of primarylithium electrochemical cells discharged at 1 mA/cm² (i.e., C/45) to a3V cutoff.

[0018]FIG. 4 is a graph depicting a comparison of the dischargeperformance of primary lithium electrochemical cells discharged ateither 0.4 mA/cm² or 1 mA/cm² to a 3V cutoff.

[0019]FIG. 5 is a graph depicting the incremental discharge capacitydistribution for a primary lithium electrochemical cell having a cathodeincluding γ-MnO₂ by Stepped Potential Electro-Chemical Spectroscopy(SPECS).

[0020]FIG. 6 is a graph depicting the discharge performance of primarylithium electrochemical cells having cathodes containing γ-MnO₂heat-treated in vacuum at either 120° C. or 150° C. in vacuo for 4 hrs.

DETAILED DESCRIPTION

[0021] Referring to FIG. 1, a lithium primary electrochemical cell 10that includes anode 12 in electrical contact with a negative lead 14, acathode 16 in electrical contact with a positive lead 18, a separator 20and an electrolyte solution. Anode 12, cathode 16, separator 20 and theelectrolyte solution are contained within housing 22. The electrolytesolution includes a solvent system and a salt that is at least partiallydissolved in the solvent system. One end of housing 22 is closed with acap 24 and an annular insulating gasket 26 that can provide a gas-tightand fluid-tight seal. Positive lead 18 connects anode 16 to cap 24. Asafety valve 28 is disposed in the inner side of cap 24 and isconfigured to decrease the pressure within battery 10 when the pressureexceeds some predetermined value. Electrochemical cell 10 can be, forexample, a cylicdrical wound cell, a button or coin cell, a prismaticcell, a rigid laminar cell or a flexible pouch, envelope or bag cell.

[0022] Anode 12 can include alkali and alkaline earth metals, such aslithium, sodium, potassium, calcium, magnesium, or alloys thereof. Theanode can include alloys of alkali or alkaline earth metals with anothermetal or other metals, for example, aluminum. An anode including lithiumcan include elemental lithium or lithium alloys, or combinationsthereof.

[0023] The electrolyte solution includes a solvent and a salt. The saltcan be an alkali or alkaline earth salt such as a lithium salt, a sodiumsalt, a potassium salt, a calcium salt, a magnesium salt, orcombinations thererof. Examples of lithium salts include LiPF₆, LiBF₄,LiAsF₆, LiClO₄, LiI, LiBr, LiAlCl₄, LiCF₃SO₃LiN(CF₃SO₂)₂,Li(C₄F₉SO₂NCN), and LiB(C₆H₄O₂)₂. The solvent can be an organic solvent.Examples of organic solvents include cyclic carbonates, chaincarbonates, ethers, esters, alkoxy alkanes, nitriles and phosphates.Examples of cyclic carbonates include ethylene carbonate and propylenecarbonate. Examples of chain carbonates include dimethyl carbonate,diethyl carbonate and ethylmethyl carbonate. Examples of ethers includediethyl ether and dimethyl ether. Examples of esters include methylpropionate, ethyl propionate, methyl butyrate and gamma butyrolactone.Examples of alkoxy alkanes include dimethoxy ethane and diethoxy ethane.Examples of nitriles include acetonitrile. Examples of phosphatesinclude triethyl phosphate and trimethyl phosphate. The electrolyte canbe a polymeric electrolyte.

[0024] The concentration of the salt in the electrolyte solution canrange from about 0.01 molar to about 3 molar, more preferably from about0.5 molar to about 1.5 molar, and most preferably about 1 molar.

[0025] Separator 20 can be formed of any of the standard separatormaterials used in lithium primary or secondary batteries. For example,separator 20 can be formed of polypropylene, polyethylene, a polyamide(e.g., a nylon), a polysulfone and/or a polyvinyl chloride. Separator 20can have a thickness of from about 0.1 millimeters to about 2millimeters, and more preferably from about 0.2 millimeters to about 0.5millimeters.

[0026] Separator 20 can be cut into pieces of a similar size as anode 12and cathode 16 and placed therebetween as shown in FIG. 1. Anode 12,cathode 16 and separator 20 can then be placed within housing 22 whichcan be made of a metal such as nickel or nickel plated steel, stainlesssteel, aluminum-clad stainless steel, or a plastic such as polyvinylchloride, polypropylene, a polysulfone, ABS or a polyamide. Housing 22containing anode 12, cathode 16 and separator 20 can be filled with theelectrolytic solution and subsequently hermetically sealed with cap 24and annular insulating gasket 26.

[0027] Cathode 16 includes an active cathode material that can undergoalkali ion insertion during discharge of battery 10. The cathode canalso include a binder, for example, a polymeric binder such as PTFE,PVDF or Viton. The cathode can also include a carbon source, such as,for example, carbon black, synthetic graphite including expandedgraphite or non-synthetic graphite including natural graphite, anacetylenic mesophase carbon, coke, graphitized carbon nanofibers or apolyacetylenic semiconductor.

[0028] The active cathode material includes lambda-manganese dioxide(γ-MnO₂), which can be synthesized by an oxidative delithiation processfrom a stoichiometric lithium manganese oxide spinel (LiMn₂O₄) precursorprepared by various synthetic methods and having differing physicalproperties. A suitable lithium manganese oxide spinel can be prepared asdescribed in, for example, U.S. Pat. Nos. 4,246,253; 4,507,371;4,828,834; 5,245,932; 5,425,932, 5,997,839, or 6,207,129, each of whichis incorporated by reference in its entirety. More particularly, thelithium manganese oxide spinel can have a formula of Li_(1+x)Mn_(2−x)O₄,where −0.02<x<+0.02. Alternatively, a suitable stoichiometric lithiummanganese oxide spinel can be obtained, for example, from Kerr-McGeeChemical LLC, Oklahoma City, Okla., Carus Chemical Co., Peru, Ill., orErachem-Comilog, Inc., Baltimore, Md.

[0029] Physical, microstructural, and chemical properties for commercialsamples of LiMn₂O₄-type spinels obtained from three different suppliersare summarized in Table 1. The x-ray powder diffraction (XRD) patternsfor the LiMn₂O₄-type spinel powders were measured using a RigakuMiniflex diffractometer using Cu K_(α) radiation. The spinel powder fromCarus Chemical has the largest refined cubic lattice cell constant,a_(o), and also has a chemical composition very close to that forstoichiometric LiMn₂O₄ spinel. The reported (e.g., ICDD PDF No. 35-0782)cubic lattice constant for stoichiometric LiMn₂O₄ spinel is 8.248 Å. Theother spinel powders from Kerr-McGee and Erachem (viz., Chemetals) haveXRD powder patterns that give refined lattice constants of 8.231 Å and8.236 Å, respectively. These latter a₀ values are more consistent withthose values typically reported for spinels having a slight excesslithium stoichiometry (i.e., Li_(1+x)Mn_(2−x)O₄, where 0<x<0.1). The a₀values for such spinels decrease linearly as x increases for x valuesbetween −0.15 and 0.25. See, for example, U.S. Pat. No. 5,425,932, whichis incorporated by reference in its entirety.

[0030] The oxidative delithiation process can include, for example, thefollowing steps:

[0031] 1. A slurry of the precursor spinel powder is formed withstirring in distilled or deionized water and adjusted to a temperaturebetween about 10 and 50° C., preferably between about 15° C. and 30° C.;

[0032] 2. An aqueous solution of an acid, such as, for example, sulfuricacid, nitric acid, hydrochloric acid, perchloric acid, toluenesulfonicacid or trifluoromethylsulfonic acid, is added to the slurry withconstant stirring at a rate to maintain a constant slurry temperatureuntil the pH of the slurry stabilizes at a value typically below about2, below about 1, or below about 0.7 but greater than about 0.5, andremains constant at this value for at least 0.75 hour (optionally,stirring can be continued for up to an additional 24 hours);

[0033] 3. The solid product is separated from the supernatant liquid,for example, by suction, pressure filtration, or centrifugation, and iswashed with aliquots of distilled or deionized water until the washingshave a neutral pH (e.g., between about 6-7); and

[0034] 4. The solid product is dried in vacuo for between 4 and 24 hoursat 30 to 120° C., preferably at 50 to 90° C., or more preferably at 60°C. to 70° C.

[0035] After processing, the dried solid typically exhibits a weightloss of about 27 wt % relative to the initial weight of the precursorLiMn₂O₄ spinel powder. The total lithium content of the stoichiometricLiMn₂O₄ spinel is about 3.8 wt %. The expected total weight loss isabout 28 wt %. The observed weight loss can be attributed to dissolutionof lithium ions that migrated to the surface of the spinel particles aswell as Mn⁺² ions from the LiMn₂O₄ spinel crystal lattice putativelyresulting from a disproportionation reaction whereby Mn⁺³ ions on thesurface of the spinel particles are converted to insoluble Mn⁺⁴ ionsthat remain on the surface and soluble Mn⁺² ions that dissolve in theacid solution according to Equation 1:

2LiMn⁺³Mn⁺⁴O₄+4H⁺→3γ-Mn⁺⁴O₂+Mn⁺²+2Li⁺+2H₂O  (1)

[0036] Maintaining the temperature of the acid solution below about 55°C. during the delithiation process can minimize the formation ofundesirable manganese oxide side-products that can form by re-oxidationby oxygen of the aqueous Mn⁺² ions. For example, the dissolution ofγ-MnO₂ can proceed by a mechanism whereby γ-MnO₂ is reduced by wateraccording to Equation 2. The resulting Mn⁺³ ions formed on the surfaceof the γ-MnO₂ particles can disproportionate according to Equation 3 toform soluble Mn⁺² ions and insoluble Mn⁺⁴. The soluble Mn⁺² ions can bereoxidized according to Equation 4 by air as well as by oxygen generatedduring the reduction of γ-MnO₂ to form undesirable manganese oxideproducts that can deposit on the surface of the remaining γ-MnO₂particles (see, Equation 2-4).

2γ-Mn⁺⁴O₂+H₂O→2Mn⁺³OOH (surface)+½O₂  (2)

2Mn⁺³OOH (surface)→γ-Mn⁺⁴O₂+Mn⁺²+2H⁺  (3)

Mn⁺²+O₂→mixtures of α-Mn⁺⁴O₂, γ-Mn⁺⁴O₂, β-Mn⁺⁴O₂, etc.  (4)

[0037] The x-ray diffraction patterns for the γ-MnO₂ powders weremeasured using a Rigaku Miniflex diffractometer using Cu K_(α)radiation. The XRD powder patterns for the various dried γ-MnO₂ powdersare consistent with that reported for γ-MnO₂ (e.g., ICDD PDF No.44-0992). See, U.S. Pat. No. 4,246,253, which is incorporated byreference in its entirety. The lattice constants, a₀, for the refinedcubic unit cells for the samples of γ-MnO₂ prepared by the methoddescribed above are given in Table 1. The a₀ values range between 8.035and 8.048 Å. T. Ohzuku et al have reported (See J. Electrochem. Soc.,Vol. 137, 1990, pp. 769) that the refined cubic lattice constant a₀, canbe correlated with the residual lithium content in the γ-MnO₂ lattice(i.e., the smaller the a₀ value, the less lithium present). FIG. 2 is agraph depicting a comparison of x-ray powder diffraction patterns forγ-MnO₂ powders prepared by either 0.75 or 16 hours of acid treatment ofa precursor spinel and the corresponding precursor spinel powder fromKerr-McGee having a nominal excess lithium stoichiometry ofLi_(1.05)Mn_(1.95)O₄. The XRD powder pattern for γ-MnO₂ isdistinguishable from that for the corresponding precursor spinel asshown in FIG. 2 for a sample of precursor spinel having a nominal excesslithium stoichiometry of Li_(1.05)Mn_(1.95)O₄ and the correspondingγ-MnO₂ acid-treated for either 0.75 or 16 hours at 15° C. by a shift inthe diffraction peak positions to higher 2-theta angles for γ-MnO₂.

[0038] The precursor spinel can have a nominally stoichiometriccomposition, for example, a composition having the formulaLi_(1+x)Mn_(2−x)O₄, wherein x is from −0.02 to +0.02, such asLi_(1.01)Mn_(1.99)O₄, from which more complete delithiation can beaccomplished, and in general, replacement of the lithium ions withprotons by an ion-exchange process, such as that shown in equation 5,can be reduced or avoided. The presence of protons in lattice sitesformerly occupied by lithium ions is theorized by the present inventorsto result in thermal instability and decreased discharge capacities forlithium cells having cathodes including such materials.

[0039] where 0.02<x<0.33

[0040] Specific surface areas of the various γ-MnO₂ powders asdetermined by multipoint nitrogen adsorption isotherms by the B.E.T.method as described by P. W. Atkins in “Physical Chemistry”, 5^(th) ed.,New York: W. H. Freeman & Co., 1994, pp. 990-2. BET measurements werefound to be substantially greater than those for the correspondingprecursor spinel powders. See, Table 1. This increase in specificsurface area is consistent with apparent increased roughness or porosityin the surface microstructure of the particles observed by comparing SEMmicrographs (10,000×) of particles of the precursor spinel, for example,and particles of the corresponding γ-MnO₂. Further, porosimetricmeasurements on a precursor spinel powder and the corresponding γ-MnO₂powder revealed that the total pore volume more than doubled afterde-lithiation to γ-MnO₂ and that the average pore size decreased bynearly 80%. TABLE 1 Precursor Spinel Spinel A Spinel B Spinel C Latticeconstant, a_(o) Spinel (Å) 8.231 8.242 8.236 Lattice constant, a_(o)λ-MnO₂ (Å) 8.048 8.035 8.041 BET SSA, Spinel (m²/g) 0.44 3.43 1.78 BETSSA, λ-MnO₂(m²/g) 4.98 8.30 7.21 Ave particle size, Spinel (μm) 12 14.628.5 Average Pore Size, Spinel (Å) 157 Average Pore Size, λ-MnO₂ (Å)36.5 Total Pore Volume, Spinel (cc/g) 0.05 λ-MnO₂ Total Pore Volume(cc/g) 0.11 Tap Density, Spinel (g/cm³) 2.10 2.08 1.96 SpinelStoichiometry, 0.06-0.08 0.01 >0.02 Li_(1+x)Mn_(2−x)O₄, x = ? TrueDensity, Spinel (g/cm³) 4.225 4.196 4.219 True Density, λ-MnO₂ (g/cm³)4.480 4.442 4.611

[0041] In certain embodiments, precursor spinels that permit preparationof γ-MnO₂ in accordance with the present invention can be selectedaccording to the following selection criteria: (1) general chemicalformula is Li_(1+x)Mn_(2−x)O₄ wherein x ranges from −0.05 to +0.05,preferably from −0.02 to +0.02, more preferably from −0.005 to +0.005;(2) BET surface area of the precursor spinel powder is between about 1and 10 m²/g; (3) total pore volume of the precursor spinel powder isbetween about 0.02 and 0.1 cubic centimeters per gram; and (4) averagepore size of the precursor spinel powder is between about 100 and 300 Å.

[0042] The thermal stability of the γ-MnO₂ powder prepared by the methodof the invention was evaluated in order to determine the effects ofvarious thermal treatments during cathode fabrication (e.g., drying,coating, pressing, etc.) on cell discharge performance. The XRD powderpatterns for a sample of γ-MnO₂ powder heated in vacuo at 120° C. for 4hours was found to be identical to that for a bulk sample of γ-MnO₂powder originally dried in vacuo at 70° C. for up to 16 hours,indicating suitable thermal stability at this temperature. The XRDpowder pattern for a sample of γ-MnO₂ powder heated in vacuo at 150° C.for 4 hours exhibited a slight broadening of the γ-MnO₂ peaks as well asthe appearance of a new broad peak at a 20 angle of about 20° indicatingthe onset of decomposition of the γ-MnO₂ phase. Heating a sample ofγ-MnO₂ powder in vacuo at 180° C. for 4 hours resulted in the completedisappearance of the characteristic γ-MnO₂ peaks and the appearance ofseveral broad peaks in the XRD pattern suggesting the formation of oneor more new phases. These poorly-resolved new peaks can be attributed tothe presence of a mixture of β-MnO₂ and ε-MnO₂ phases.

[0043] In addition to evaluating the thermal stability of the γ-MnO₂powder, the thermal stability of γ-MnO₂ in pressed composite cathodesalso containing a conductive carbon and a binder was evaluated. XRDpatterns for pressed composite cathodes after heating for 4 hours at120° C. showed a broadening of the γ-MnO₂ peaks as well as theappearance of several new, broad, weak peaks attributed to the 6-MnO₂phase indicating the onset of decomposition of the γ-MnO₂ phase. Thus,γ-MnO₂ in the pressed composite cathode appears to start decomposing atan even lower temperature than γ-MnO₂ powder alone. In XRD patterns forcathodes heated at 150° C. or 180° C., all of the peaks attributed tothe γ-MnO₂ phase disappeared completely, and only broad peakscharacteristic of the ε-MnO₂ phase were present. Furthermore, unlike thecase of γ-MnO₂ powder, no peaks for the β-MnO₂ phase could be discernedin the XRD pattern for a composite cathode heated at 180° C.

[0044] Lithium primary cells including composite cathodes containingγ-MnO₂ were prepared according to the following representative examples.

EXAMPLE 1

[0045] Approximately 120 g of a nearly stoichiometric spinel B having anominal composition of Li_(1.01)Mn_(1.99)O₄ (Carus Chemical Co.) wasadded with stirring to about 200 ml distilled water to form an aqueousslurry that was cooled to 15° C. 6M H₂SO₄ was added dropwise withconstant stirring until the pH of the slurry stabilized at about 0.7.The slurry was stirred for an additional 20 hours at pH 0.7. The rate ofacid addition was adjusted so as to maintain the temperature of theslurry at 15° C. The solid was separated from the liquid by eitherpressure or suction filtration through a non-woven, spun-bondedpolyethylene film (Dupont, Tyvek) and washed with aliquots of distilledwater until the washings had a neutral pH (e.g., a pH of about 6). Thesolid filtercake was dried in vacuo for 4-16 hours at 70° C. The weightof the dried γ-MnO₂ product was about 87 g, which corresponds to aweight loss of about 27.5%.

[0046] Samples of dried ?-MnO₂ powder were mixed with carbon black(Chevron SAB/C55) as a conductive additive and PTFE powder (Dupont 601A)as a binder in a weight ratio of 60:10:30 in a laboratory blender toform a cathode mix. Portions (˜0.5 g) of the cathode mix were pressedinto composite cathode disks about 17.5 mm in diameter (i.e., ˜2.5 cm²in area) and inserted into stainless steel test cells having aneffective internal diameter of 17.5 mm thereby simulating a typicallithium coin cell. Such cells also have limited electrolyte volume,electrodes in close geometric proximity, and a positive pressure (e.g.,2 kg/cm²) applied to the electrodes by a coil spring inside the cell.The design of the test cell is similar to that described by Geronov etal., in J. Electrochem. Soc., Vol. 137, No. 11, 1990, pp. 3338-3344,which is incorporated herein by reference in its entirety. A diskpunched from lithium metal foil 1 mm thick served as the anode. Theelectrolyte solution was 1M LiPF₆ in 1:1, v/v EC:DMC (EM Industries,ZV1020) as typically used in secondary lithium-ion cells. A separatorsheet in the form of a disk was saturated with electrolyte solution andplaced on top of the cathode disk. Additional electrolyte was added toensure complete wetting of the cathode disk.

[0047] The spring-loaded cells of Example 1a with cathodes containingγ-MnO₂ prepared from the spinel B were discharged at a nominal constantcurrent of 1 mA corresponding to a current density of 0.4 mA/cm² and anominal discharge rate of about C/45. These cells were discharged tocutoff voltages of 3.5V or 3V. Gravimetric or specific (viz., mAh/g)discharge capacities for the cells of Example 1a of about 135 mAh/g to3V and 133 mAh/g to 3.5V were obtained. See Table 2. Also, the dischargecurve for the cells of Example 1 a exhibited two distinct, flat plateausat about 4.05V and about 3.95V, as shown in FIG. 3.

[0048] Another cathode mix was prepared by mixing γ-MnO₂ dried at 70° C.for 4 hours with carbon black (viz., Chevron SAB/C55) as a conductiveadditive and of PTFE powder (e.g., DuPont 601A) as a binder in a weightratio of 75:10:15 in a laboratory blender. Portions (˜0.5 g) of thecathode mix were pressed into cathode disks that were inserted intoseveral spring-loaded cells. The spring-loaded cells of Example 1b withcathodes containing γ-MnO₂ prepared from spinel B were discharged at aconstant current of 2.5 mA, corresponding to a current density of 1mA/cm² and a nominal discharge rate of C/25. The cells were dischargedto cutoff voltages of 3.5V or 3V. Gravimetric discharge capacities ofabout 137 mAh/g to 3V and 134 mAh/g to 3.5V were obtained for the cellsof Example 1b. See Table 2.

COMPARATIVE EXAMPLE 1

[0049] Samples of γ-MnO₂ were prepared in the same manner as describedin Example 1 except that spinel A having the nominal compositionLi_(1.06)Mn_(1.94)0₄ (Kerr-McGee, #210) was employed. A cathode mixprepared by mixing γ-MnO₂ dried at 70° C. for 4 hours with carbon black(viz., Chevron SAB/C55) as a conductive additive and PTFE powder (viz.,DuPont 601A) as a binder in a weight ratio of 60:10:30 in a laboratoryblender. Portions (˜0.5 g) of the mix were pressed into compositecathode disks that were inserted into several spring-loaded cells. Thecells were discharged at a constant current of 1 mA to a cutoff voltageof 3V or 3.5V. The average open circuit voltage for freshly assembledcells was about 4.15V. Gravimetric discharge capacities of about 116mAh/g to 3V and 113 mAh/g to 3.5V were obtained. See Table 2. Similar tothe discharge curves for the cells of Example 1, the discharge curvesfor cells of Comparative Example 1 exhibited two distinct, flat plateausat about 4.05V and about 3.95V as shown in FIG. 4.

COMPARATIVE EXAMPLE 2

[0050] Samples of γ-MnO₂ were prepared in the same manner as describedin Example 1 except that spinel B having a nominally stoichiometriccomposition (Erachem/Chemetals, LMO-800E) was employed. A cathode mixprepared by mixing γ-MnO₂ dried at 70° C. for 4 hours with carbon black(viz., Chevron SAB/C55) as a conductive additive and PTFE powder (viz.,DuPont 601A) as a binder in a weight ratio of 85:5:10 in a laboratoryblender. Portions (˜0.5 g) of the mix were pressed into compositecathode disks that were inserted into several spring-loaded cells. Thecells were discharged at a constant current of 2.5 mA to a cutoffvoltage of 3V or 3.5V. The average open circuit voltage for freshlyassembled cells was about 4.17V. Gravimetric discharge capacities ofabout 120 mAh/g to 3V and 105 mAh/g to 3.5V were obtained. See Table 2.Unlike the discharge curves for the cells of Example 1, the dischargecurves for the cells of Comparative Example 2 did not have two distinct,flat plateaus at about 4.05V and about 3.95V, but instead exhibited asloping discharge profile as shown in FIG. 4.

EXAMPLE 2

[0051] Samples of γ-MnO₂ composite cathodes were prepared in the samemanner as described in Example 1 except that they were heat-treated invacuo at 120° C. for 4 hours (Ex. 2a) or 16 hours (Ex. 2b) or at 150° C.for 4 hours (Ex. 2c) before being inserted in lithium spring-loadedcells. These cells were discharged continuously at a constant current of1 or 2.5 mA to a cutoff voltage of 3.5V and the resulting gravimetricdischarge capacities given in Table 2. A comparison of the 4V plateaudischarge capacities with those obtained for cells having γ-MnO₂cathodes dried at 70° C. with no further heat-treatment clearlyindicates that heat-treatment at 120° C. for 16 hours substantiallydegrades the 4V capacity. Heat-treatment at 150° C. for only 4 hoursresults in complete loss of 4V capacity. This loss of 4V capacity isdramatically depicted in FIG. 6.

[0052] Typical discharge curves for cells containing the γ-MnO₂ samplesprepared from various commercial spinels are shown in FIG. 4. Lithiumcells with composite cathodes containing γ-MnO₂ from spinel C dischargedto a 3V cutoff at a nominal rate of about 1.0 mA/cm² gave specificcapacities of about 120 mAh/g. Under similar discharge conditions, cellswith composite cathodes containing γ-MnO₂ prepared from the spinel Bgave a substantially greater specific capacity of about 135 mAh/g. Cellswith composite cathodes containing γ-MnO₂ prepared from spinel A gaveeven lower specific capacities of about 115 mAh/g even at asubstantially lower nominal discharge rate of about 0.4 mA/cm². Further,the discharge curves for cells containing γ-MnO₂ prepared from spinel Aand spinel B both exhibit two distinct, relatively flat plateaus atabout 4.05V and 3.95V similar to those typically observed for anelectrochemically-charged spinel. In contrast, discharge curves forcells containing γ-MnO₂ prepared from spinel C exhibit a single slopingplateau in the 4V region with an average CCV of about 3.9V. Based on thedischarge data for cells shown in FIG. 4, the γ-MnO₂ prepared fromspinel B clearly provides superior low-rate discharge performancecompared to γ-MnO₂ derived from either spinel A or spinel C. TABLE 2Heat Discharge Ave Capacity Capacity Ex. Treatment Rate OCV to 3.5 V to3 V No. Spinel (° C./hours) (mA) (V) (mAh/g) (mAh/g) 1a B 70/4 1 4.18133 135 1b B 70/4 2.5 4.14 134 137 C1 A 70/4 1 4.15 113 116 C2 C 70/42.5 4.17 105 120 2a B 120/4  1 4.16 125 — 2b B 120/16 2.5 4.14 106 2c B150/4  1 3.78 0.5

[0053] In addition, the incremental discharge capacity for aspring-loaded lithium cell having a cathode containing γ-MnO₂ of Example1 was determined using the Stepped Potential Electro-ChemicalSpectroscopy (SPECS) method described in detail by Bowden et al. in ITELetters on Batteries, Vol. 1, No. 6, 2000, pp. 53-64, which isincorporated herein by reference in its entirety, and depicted in FIG.5. The sharp peaks denoting the incremental capacity resulting from theinitial discharge at a nominal sweep rate of 5 mV/hr of the γ-MnO₂cathode material to a cutoff of 3.3V are centered at about 4.05V (peakA) and 3.9V (peak A′) indicating the suitability of this material foruse in a nominally 4V lithium cell. The relatively low incrementalcapacity of the broad peak at about 4.15V (peak B) obtained duringrecharge of the cell to a cutoff of 4.2V and the lack of any incrementalcapacity for the 4.05V and 3.9V peaks and the broad, low capacity peakcentered at about 3.7V (peak C) during the second discharge of the cellto a 3.3V cutoff demonstrate the poor rechargeability of the γ-MnO₂composite cathode. However, the γ-MnO₂ composite cathode is particularlysuitable for use in primary lithium cells having a lithium metal anode.The inability to recharge a cell having a composite cathode containingγ-MnO₂ minimizes potential safety hazards related to re-depositinglithium metal at the anode during recharge and provides a substantialadvantage for the 4V lithium primary cells compared to secondary lithiumcells having lithium metal anodes.

[0054] Discharge curves for spring-loaded cells containing γ-MnO₂cathodes heat-treated at 120° C. and 150° C. are depicted in FIG. 6. Thecells were discharged at a nominal rate of 0.4 mA/cm² to a 2.0V cutoff.The cells containing the γ-MnO₂ cathodes that were heat-treated at 120°C. gave a capacity of 125 mAh/g to a 3.5V cutoff, which is somewhat lessthan those cells containing non-heat-treated cathodes (e.g., 133 mAh/g).See Table 2. The lower capacity presumably is a consequence of somedecomposition of the γ-MnO₂ phase during heat-treatment at 120° C. asevidenced in the XRD patterns. These cells gave a total capacity ofabout 235 mAh/g to 2V. The cells containing γ-MnO₂ cathodes heat-treatedat 150° C., which actually contained predominantly ε-MnO₂ phase based onthe XRD pattern, had an OCV of ˜3.7-3.8 V, a CCV of ˜2.8 V, and alsogave a total capacity of about 240 mAh/g to a 2V cutoff. Thus, itappears that all the capacity originally present on the 4 V plateau wasshifted down to the 3 V plateau as a result of the heat treatment at150° C. Therefore, it is necessary to keep processing temperatures belowabout 100° C. during drying, heat-treatment, and cathode fabrication andcell assembly processes in order to maintain the capacity on the 4Vplateau.

[0055] A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, oxidative delithiation of the precursor spinel can be performedusing a variety of aqueous oxidizing agents including, for example, anaqueous solution of sodium or potassium peroxydisulfate, sodium orpotassium peroxydiphosphate, sodium perborate, sodium or potassiumchlorate, sodium or potassium permanganate, cerium (+4) ammonium sulfateor nitrate, or sodium perxenate, or ozone gas bubbled through acidicwater. Non-aqueous oxidizing agents include, for example, nitrosonium ornitronium tetrafluoroborate in acetonitrile, nitrosonium or nitroniumhexafluorophosphate in acetonitrile, or oleum (i.e., SO₃/H₂SO₄) insulfolane. Using an aqueous chemical oxidant such as peroxydisulfate,ozone or a non-aqueous oxidizing agent to oxidize the Mn⁺³ ions to Mn⁺⁴ions in the LiMn₂O₄ spinel can result in substantially less manganesebeing lost by dissolution than in the disproportionation process(Equation 1) taking place during treatment with an aqueous acidsolution.

[0056] Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A primary lithium electrochemical cellcomprising: a cathode including lambda-manganese dioxide; an anodeincluding lithium; a separator between the anode and the cathode; and anelectrolyte contacting the cathode, the anode and the separator, whereinthe cell has an average closed circuit voltage of about between about3.8 and 4.1V and a specific discharge capacity to a 3V cutoff of greaterthan 130 mAh/g at a nominal discharge rate of 1 mA/cm².
 2. Theelectrochemical cell of claim 1, wherein the cell has a 3V cutoff ofgreater than 135 mAh/g.
 3. The electrochemical cell of claim 1, whereinthe cell has a 3V cutoff of 140 mAh/g or greater.
 4. The electrochemicalcell of claim 1, wherein the lambda-manganese dioxide is maintained at atemperature of less than 150° C. during processing or cathodefabrication.
 5. The electrochemical cell of claim 1, wherein the cathodecontaining the lambda-manganese dioxide is maintained at a temperatureof 120° C. or less during processing or fabrication.
 6. Theelectrochemical cell of claim 1, wherein the lambda-manganese dioxidehas a BET surface area of greater than 4 m²/g.
 7. The electrochemicalcell of claim 1, wherein the lambda-manganese dioxide has a BET surfacearea of greater than 8 m²/g.
 8. The electrochemical cell of claim 1,wherein the lambda-manganese dioxide has a total pore volume of from0.05 to 0.15 cubic centimeters per gram.
 9. A primary lithiumelectrochemical cell comprising: a cathode including lambda-manganesedioxide having a total pore volume of greater than 0.11 cubiccentimeters per gram, and the lambda-manganese dioxide has a BET surfacearea of greater than 8 m²/g, wherein the lambda-manganese dioxide ismaintained during processing at a temperature of 120° C. or less; ananode including lithium or a lithium alloy; a separator between theanode and the cathode; and an electrolyte contacting the cathode, theanode and the separator, wherein the cell has an average closed circuitvoltage of about 4V, a specific discharge capacity to a 3V cutoff ofgreater than 130 mAh/g at a nominal discharge rate of 1 mA/cm².
 10. Theelectrochemical cell of claim 9, wherein the cell has a 3V cutoff of 135mAh/g or greater at a nominal discharge rate of 0.4 mA/cm².
 11. A methodof preparing lambda-manganese dioxide comprising: contacting water witha compound of the formula Li_(1+x)Mn_(2−x)O₄, wherein x is from −0.02 to+0.02; adding an acid to the water and compound until the water has a pHof 1 or less; separating a solid from the water and acid; and drying thesolid at a temperature of 120° C. or below to obtain thelambda-manganese dioxide.
 12. The method of claim 11, wherein thecompound has a BET surface area of between 1 and 10 m²/g.
 13. The methodof claim 11, wherein the compound has a spinel-type crystal structure.14. The method of claim 11, wherein the solid is dried at a temperaturebetween 30° C. to 90° C.
 15. The method of claim 11, wherein the solidis dried at a temperature between 50° C. and 70° C.
 16. The method ofclaim 11, wherein x is from −0.005 to +0.005.
 17. The method of claim11, wherein contacting water and the compound includes forming a slurry.18. The method of claim 17, wherein the slurry is maintained at atemperature below 50° C.
 19. The method of claim 11, wherein the acidsulfuric acid, nitric acid, perchloric acid, hydrochloric acid,toluenesulfonic acid or trifluoromethylsulfonic acid.
 20. The method ofclaim 17, wherein the temperature of the slurry is held substantiallyconstant during the addition of acid.
 21. The method of claim 11,wherein the pH is 0.7 or less.
 22. The method of claim 11, wherein theacid has a concentration of between 1 and 8 molar.
 23. The method ofclaim 11, further comprising washing the solid separated from the liquidphase with water until the washings have a pH of between 6 and
 7. 24. Amethod of manufacturing an electrochemical cell comprising: providing anpositive electrode including a lambda-manganese oxide; and forming acell including the electrode and a lithium negative electrode, whereinthe cell has a closed circuit voltage of about 4V and a specificdischarge capacity at a nominal discharge rate of 1 mA/cm² to a 3Vcutoff of greater than 120 mAh/g.
 25. The method of claim 24, whereinproviding the electrode includes preparing lambda-manganese dioxide by amethod comprising: contacting water with a compound of the formulaLi_(1+x)Mn_(2−x)O₄, wherein x is from −0.02 to +0.02; adding an acid tothe water and compound until the water has a pH of 1 or less; separatinga solid from the water and acid; and drying the solid at a temperatureof 120° C. or below to obtain the lambda-manganese dioxide.